One portion of the solar spectrum comprises wavelengths of electromagnetic energy which range between about 290 and 3,000 nanometers (nm). This range may be divided into different regions, namely: (1) the ultraviolet region (290-400 nm), (2) the visible region (400-760 nm) and (3) the near-infrared region (&gt;760 nm). The ultraviolet region has, moreover, been arbitrarily divided into three bands, referred to as the UVA, UVB and UVC bands.
The UVB band extends from 290 to 320 nm. It is the principal cause of the sunburn reaction and it is also the most effective in stimulating the tanning reaction in the skin. UVC radiation (200-290 nm) from the sun does not reach the surface of the earth, although one can encounter radiation in this range from artificial sources such as germicidal lamps and high and low pressure mercury arc lamps. For purposes of the present invention however, protection against UVC radiation is generally not a major concern, i.e., in contrast to the dangers posed by UVA and UVB radiation. The UVA band, which extends from 320-400 nm, can also cause the tanning reaction. UVA radiation can also cause sunburns, but its capacity to do so is less than that of UVB radiation.
The amount of UVA radiation exposure, however, is increasing. This is due to the fact that most sunscreens effectively block only UVB radiation. As stated above, UVB radiation is more capable than UVA radiation of causing the tanning and burning reactions. Therefore, if one is using a sunscreen that blocks UVB radiation he/she will tend to stay in the sun for an extended period of time because the immediate effects of the sun tan/burn are not evident. The problem is that UVA is still penetrating the skin and although it is not causing any immediately obvious effects, it is causing long term damage. In recent years, it has been well documented that UVA radiation, like UVB radiation, is harmful to the skin. In fact, current data reveal that solar radiation containing these wavelengths is the chief cause of skin cancer, which presently accounts for 30-40% of all new cancers each year. In the United States alone, 500,000 new cases of skin cancer will be reported this year and the number is expected to keep rising in the future. UVA radiation has been shown to promote skin cancer by inhibiting enzymes that repair cells damaged by UVB radiation. UVA radiation also penetrates more deeply into the skin than UVB radiation and causes changes in blood vessels and premature aging of the skin, thus adding to the damage produced by UVB rays (see, e.g., Hurwitz, Sidney, "The Sun and Sunscreen Protection: Recommendations for Children", Dermatol. Surg. Oncol; 14:6 (June 1988) p. 657). The goal of any sunscreen should thus be to protect the user from both UVA and UVB radiation with a minimum of side effects. This end has not been adequately achieved with the use of presently available sunscreen products.
Sunscreen products can be grouped into two broad categories, i.e., (1) topical sunscreens and (2) oral sunscreens. The present invention focuses upon the topical sunscreens, which can be further differentiated into two subcategories, namely (1) chemical sunscreens and (2) physical sunscreens.
Chemical sunscreens contain from about 3 to about 26% of one or more UV-absorbing chemicals. When applied to the surface of the skin as a thin film, i.e., about 10-15 .mu.m in thickness, these chemicals act as a filter to diminish the penetration of UV radiation to the cells of the epidermis. These sunscreens are typically applied in a cream, oil, lotion, alcohol or gel vehicle and they are usually colorless because they do not contain any visible light-absorbing chemicals. The most widely used chemical sunscreens contain, for example, para-aminobenzoic acid (PABA), PABA esters (glyceryl PABA, amyldimethyl PABA and octyldimethyl PABA), benzophenones (oxybenzone and sulisobenzone), cinnamates (octylmethoxy cinnamate and cinoxate), salicylates (homomethyl salicylate) and anthranilates. To date, more than twenty-one such chemicals have been approved by the United States Food and Drug Administration as "safe and effective" agents in protecting skin against sunburn (see, e.g., Pathak, Madhu, "Sunscreens: Topical and Systemic Approaches for Protection of Human Skin Against Harmful Effects of Solar Radiation", Continuing Medical Education Series, J. Am. Acad. Dermat., 7:3 (September 1982) p. 285, 291).
Questions have recently been raised, however, by the medical profession as to whether the chemical components of these sunscreens are indeed inert and further, whether repeated use of such sunscreens can result in significant transdermal absorption of these chemicals. Because chemical sunscreens are applied topically in relatively high concentrations (i.e., up to 26%), contact and photocontact sensitization can occur, as well as hypersensitivity, i.e., photoallergic reactions (see Drumgoogle et al., "Sunscreening Agent Intolerance: Contact and Photocontact Sensitization and Contact Urticania", J. Am. Acad. Dermatol., 1990:22, p. 1068).
Physical sunscreens, on the other hand, comprise particles of a relatively physiologically inert sunblock, i.e., UV-absorbing, compound typically suspended in a cream or lotion. Materials frequently utilized for this purpose include kaolin, talc and two metal oxides, i.e., titanium dioxide and zinc oxide. The latter two compounds are not associated with the inflammatory reactions noted above.
The physical sunscreen products are, however, typically messy and occlusive. Moreover, they additionally form a visible, colored (e.g., white) layer on the surface of the skin which is cosmetically unacceptable to many who are in need of sunscreen protection. This causes many such individuals to forego the use of these products. The color of these compositions is attributable to the optical properties of the particles from which these materials are formed. These properties are at least partially dependent upon the size of these particles, which typically have a fairly "standard" range of diameters, measured in tenths of a micron (i.e., about 0.1-0.9.mu.).
In addition, presently available physical sunscreens are not easily washed off of the user's body. Instead, they typically melt off with the heat of the sun, thus incidentally staining or otherwise discoloring the user's clothing. Moreover, because they are applied as relatively thick films (20-50 .mu.m), use of these products may also promote undesirable skin conditions, including miliaria, a skin disease caused by an inflammation of the sweat glands, and folliculitis, an inflammation of the hair follicle. As such, these physical sunscreen products are deemed cosmetically unacceptable by a large class of image conscious persons, which primarily includes young people. Unfortunately, this same group is the exact population that needs solar protection the most. It has been stated that proper use of sunscreens prior to the age of 18 would prevent 80% of skin cancers (see, e.g., Taylor et al., "Photoaging/Photodamage and Photoprotection", 22 J. Am. Acad. Dermatol., 9 (1990).
In one variant of the "typical" prior art physical sunblocks described above, certain commercial sunscreen products containing titanium dioxide are made with what is known as "micronized" or "large surface area" particles of the metal oxide. These particles have a diameter an order of magnitude smaller (i.e., measuring about 0.01.mu.) than the "standard" sized particles (measuring about 0.1-0.9.mu.) described above. One drawback to the use of this material, however, is that titanium dioxide absorbs neither as much UV-radiation nor transmits as much visible radiation as, for example, zinc oxide, which is utilized by applicants in the present invention (see, e.g., Brown, Harvey E., Zinc Oxide: Properties and Applications, pp. 11-12, FIG. 2-4 (1976)). Thus, although the use of micronized titanium dioxide particles does render the resultant product smoother and less occlusive, it does not obviate the main drawback faced with the use of this material, i.e., its comparatively lower effectiveness (in contrast to Zn0) as a sunblock agent. Titanium dioxide-based products are also more opaque than those formed with the zinc oxide of the present invention, which is due to the fact that the crystalline structure of the titanium dioxide material renders it only partially transparent to visible wavelengths of light and thus not generally as acceptable for cosmetic use.
Although it has been known to form micronized particles of zinc oxide for very specialized uses in the rubber industry, these particles contain substantial quantities (i.e., greater than about 200 ppm) of trace metals such as lead, mercury, arsenic and cadmium. The potential dangers to human health caused by exposure to these materials is well-documented. Thus, such zinc oxide particles containing these levels of trace metals are not acceptable for topical application to human skin.
Greater public awareness of the harmful effects of exposure to excessive solar radiation has therefore resulted in an increased use of sunscreen products by the public, coupled with a call for improved sunscreen materials free of the drawbacks described above by those whose livelihood and/or leisure activities cause them to be exposed to any substantial amounts of solar radiation.